JEE Chemistry Hydrocarbons Notes Set 09

Electrophilic Aromatic Substitution: Mechanism

The pi cloud of benzene is stable (aromatic), so only a strong electrophile E+ can attack it. In the slow first step E+ bonds to one ring carbon and gives a resonance-stabilised carbocation, the sigma complex or arenium ion. This carbon is now sp3, so the ring is no longer aromatic and this step is highly endothermic.
In the fast second step a base removes the proton from the sp3 carbon, aromaticity returns and the product is Ar-E. The overall change is replacement of ring H by E, so the reaction is electrophilic aromatic substitution (SEAr).
The energy profile has two transition states with the sigma complex as an intermediate between them; the first transition state is the higher one, so the first step is rate determining. Many reagents that add readily to alkenes (peroxy acids, diborane, bromine without a catalyst) do not react with benzene, because they are not electrophilic enough.

Directing and Activating Effects of Substituents

Groups with a lone pair or a +I effect release electrons, stabilise the sigma complex for ortho or para attack and are ortho-para directing and activating. Electron-withdrawing groups (-M, -I) destabilise it, slow the reaction and direct to the meta position, where the positive charge is least next to the substituent.
Halogens are the exception: the -I effect makes the ring less reactive, but the +M effect of the lone pair decides the position, so they are deactivating and ortho-para directing.
Two substituents together: when an activating and a deactivating group are present the activating group decides; when both are activating, or both deactivating, the stronger one decides. There is little substitution between two groups that are meta to each other because of steric crowding.

Halogenation and Nitration

Halogenation: chlorine and bromine react with benzene in the presence of a Lewis acid (FeX3, AlCl3), which polarises X-X to generate a stronger electrophile: \( \ce{C6H6 + Br2 ->[FeBr3] C6H5Br + HBr} \). Fluorination is too violent to control and iodination is too slow with an unfavourable equilibrium, so they are rarely done directly.
Nitration: the nitrating mixture of conc. HNO3 and conc. H2SO4 generates the nitronium ion: \( \ce{HNO3 + 2H2SO4 <=> NO2+ + H3O+ + 2HSO4-} \). Nitric acid acts as a base here, and the sulphuric acid raises the concentration of NO2+. NO2+ attacks benzene to give nitrobenzene.

Sulphonation and Friedel-Crafts Reactions

Sulphonation: SO3 in concentrated sulphuric acid (oleum) is the electrophile, produced as \( \ce{2H2SO4 <=> SO3 + H3O+ + HSO4-} \). Unlike other substitutions it is reversible: the sulphonic acid group is removed again by heating with dilute acid or steam, because the arenium ion can lose either H+ or SO3.
Deuterium isotope effect: with C6D6 the loss of D+ from the sigma complex is slower than loss of H+, so the sulphonation rate drops. This is the only common EAS where the C-H breaking step is partly rate determining.
Friedel-Crafts alkylation: an alkyl halide with AlCl3 forms a carbocation (or polarised complex), which attacks the ring. Friedel-Crafts acylation: an acid chloride or anhydride with AlCl3 forms the acylium ion RCO+, which gives an aryl ketone; the product ketone forms a complex with AlCl3, so more than one equivalent of the catalyst is needed.
Limitations: (1) very reactive rings such as aniline (the amine complexes with AlCl3) and strongly deactivated rings such as nitrobenzene, benzonitrile and benzoic acid do not react; (2) alkylation can go on to polyalkylation because the alkyl group activates the ring, whereas acylation stops at one acyl group because the C=O deactivates it; (3) alkylation may give rearranged products through carbocation shifts, while acylation never rearranges; (4) diacid chlorides and diketones can also react with benzene.